(1) Field of the Invention
The present invention relates to a male or female threaded tubular element of a threaded tubular connection which is particularly capable of resisting both static and cyclic stresses.
The present invention also relates to a threaded tubular connection which is particularly suitable for resisting both static and cyclic stresses.
(2) Description of Related Art
Threaded tubular connections comprise a male threaded element at the end of a first pipe and a female threaded element at the end of a second pipe which may be a great length pipe or a coupling. Such threaded connections are used in particular to constitute casing strings or production strings or drillpipe strings for hydrocarbon wells or for similar wells such as for example geothermal wells.
In its API specification 5B, the American Petroleum Institute (API) defines threaded connections between casing pipes or between production pipes in particular with tapered threadings with trapezoidal or round triangular threads.
Other types of threaded connections are also known which use straight or tapered two-step thread: see, for example, U.S. Pat. Nos. 4,521,042 and 5,687,999.
Until recently, casing pipes or production pipes had essentially to be capable of resisting different combinations of static stresses (tension, axial compression, plane bending, internal or external pressure) despite their limited thickness resulting from the need, to be able to exploit a deep well, to insert a variety of columns of different diameters one into another.
In contrast, drillpipes, which are only used to drill wells, are subjected to substantial cyclic stresses (dynamic ones) but are not subjected to size limitations, since a single string of a given diameter is downhole at a given time.
If not strictly limited, cyclic stresses lead during operation to fatigue ruptures which start at the thread roots, generally on the side of the load flanks which are beneath load in operation, more particularly at the last engaging threads of each of the threaded elements of the drillpipes.
In the remainder of the present document, the term xe2x80x9cfirst threadsxe2x80x9d means the threads which, in a longitudinal cross section passing through the axis of the threaded element, are located on the free end side of the threaded element. As a result, the last threads are those located at the other end of the threading.
The term xe2x80x9cengaging threadsxe2x80x9d firstly means the threads of a threaded tubular connection which transfer the load from one threaded tubular element to the mated threaded tubular element.
When the threaded connection is subjected to tensile loads, the engaging threads are those where the load flanks are in contact and transfer the load from one threaded element to the mated threaded element.
By extension, the term xe2x80x9cengaging threads of a threaded tubular elementxe2x80x9d as used in the present document means the threads intended to transfer the load to the corresponding threads of a mated threaded tubular element when these two threaded tubular elements are connected to constitute a threaded tubular connection.
The position of the engaging threads of a threaded tubular element is known from the design of the threaded element. This is a theoretical factor defined by the nominal dimensions of the threaded elements to be connected.
The position of the last or first engaging threads can thus be perfectly defined for a threaded tubular element intended for a threaded connection.
However the problem of fatigue behavior is now not only encountered in drillpipes but also for production pipe strings for certain hydrocarbon wells.
The threaded tubular connections which enable such strings to be produced must be capable of tolerating both high static stresses and cyclic stresses.
Such demands on stress behavior are now being encountered in underwater strings connecting the seabed to offshore hydrocarbon exploitation platforms.
Such columns of pipes, known to the English-speaking skilled person as xe2x80x9crisersxe2x80x9d, are indeed subjected to cyclic stresses caused in particular by currents which induce vibrations in the column, by waves, by tides and by possible displacement of the platforms themselves, all stresses which induce mainly cyclic bending stresses and/or tension-compression stresses.
Such demands on stress behavior are also encountered in onshore wells, in particular when dropping rotating pipes in order to cement wells in the very frequent case of wells which deviate from the vertical and have bends; the rotating dropping produces then a rotative bending.
For this reason, improvements to threaded tubular connections for casing pipes, for production pipes or for risers have been sought in order to increase their fatigue strength.
The prior art for threaded tubular or non-tubular connections (screw-nut type, for example) proposes means for improving the fatigue strength of threaded connections subjected to axial tensile loads which can vary cyclically.
patent application WO 00/06931 describes a female threaded element for a threaded tubular connection the outside peripheral surface of which is tapered and has a diameter which decreases as the female free end gets nearer so that the material thickness beneath the threading is reduced at the level of the first threads. There results a great sensitiveness to shocks of the female free end, which is very slim.
Many documents, including U.S. Pat. No. 5,779,416 and patent applications JP 04 157 280 and JP 04 157 283, use a groove in the form of a U beyond the last male engaging threads in the non threaded portion under tension of the male threaded element. Such a groove has the major disadvantage of reducing the critical cross section of the threaded connection which is the most stressed under axial tension and consequently of reducing the tensile static performances of the threaded connection.
U.S. Pat. No. 3,933,074 describes a nut for a bolted connection where the internal threading is interrupted at the first engaging threads by a plurality of axial channels in hollow regularly disposed on the periphery of the threading in order to displace the maximal transfer zone for axial tensile stress between the screw and nut from the first female engaging thread towards the middle of the axial length of the bolt.
Those channels the length of which can reach half the length of the threading and the depth of which can reach up to 80% of the height of the thread, increase the flexibility of the first engaging threads but reduce by about 20% the bearing surface of the threads in the zone where they are produced, which is a disadvantage when a high resistance to static stress is sought and when a threaded tubular connection is sought the interior and exterior of which are sealed.
Moreover, solutions as regard bolts in which the nuts are bearing on the side of the first threads against the screw casing-head (on the side of the last threads of the screw) are not necessarily directly applicable to threaded tubular connections.
Patent FR 1 317 815 describes an annular groove with the profile of a basin comparatively not very deep formed on the external peripheral surface of a female threaded element of a drill pipe.
In the figures of this patent, the groove is disposed in the middle of the threading and has no effect on the wall at the level of the first or last threads. It enables to spread out the concentrations of stresses on the whole of the male threading while increasing the stresses at the level of the threads which are situated beneath the groove about the middle of the threading.
Nevertheless this patent does not tell to which stresses the stress fields shown in the figures (torsion, tension, compression, bending) correspond. They seem to be stresses simply resulting from the made up state of the threaded elements of the drillpipes.
It will also be noted, still in the figures of this patent, that the groove has a flat bottom parallel to the axis of the threaded connection and steep flanks, which are roughly perpendicular with respect to the groove bottom and to the external peripheral surface.
Patent applications JP 58-187684 and EP 0 032 265 describe a male threaded element which is provided with an annular groove with a basin profile formed on the internal peripheral surface of the threaded element at the level of the non-threaded lip at a free end or essentially formed at the level of said lip.
In those two documents, a lip has been provided for improving other characteristics of a threaded tubular connection than fatigue strength (galling strength, stress corrosion strength, locking of the threaded elements in position) and in those two documents nothing suggests that a groove formed under the male lip (and possibly slightly protruding under the first two male threads in the case of the Japanese document) might improve fatigue strength of a threaded tubular connection.
The aim of the present invention is to produce a male or female threaded tubular element for threaded tubular connections, which is particularly resistant both to:
a) static stresses, in particular axial tension, axial compression, bending, torsion, internal or external pressure, dislodging during connection, either simple or combined (for example tension+internal pressure);
b) cyclic stresses, in particular bending stresses and tension-compression stresses.
In the remainder of the present document, such a threaded element will be described as having an anti-fatigue profile.
The present invention also aims to ensure that the threaded tubular element of the invention can be formed with all types of threadings: tapered, straight, straight-tapered combinations, with one or more steps, with trapezoidal or triangular threads, which may be interfering or non-interfering; non interfering threadings can be, for example, of the type described in European patent application EP 0 454 147 with simultaneous contact of the two flanks with those of the mated thread (known as xe2x80x9crugged threadxe2x80x9d), with an axial interference fit of the type described in document WO 00/14441, or of the wedge type with a varying width as described, for example, in US Re 30 647.
A further aim is that the threaded element can be produced and inspected easily.
The threaded element of the invention must be able to be used to constitute threaded connections for strings of hydrocarbon production pipes, for well casings or for underwater exploitation xe2x80x9crisersxe2x80x9d or for similar uses.
A still further aim is to produce threaded tubular connections which are sealed, in particular gas tight, even under cyclic stresses.
In a variation, the threaded element of the invention must be able to be used in drillpipe strings.
A still further aim is to produce a threaded tubular connection in which only one of the threaded elements, for example the female element, has been modified to resist cyclic stresses but which accommodates a non-modified mated threaded element.
In a variation, both threaded tubular elements of the threaded tubular connection have been modified to resist cyclic stresses.
The male or female threaded tubular element with an anti-fatigue profile is formed at the end of a pipe and comprises a male threading on its external peripheral surface or a female threading on its internal peripheral surface depending on whether the threaded tubular element is of the male type or of the female type.
Said threaded tubular element is intended for a connection by screwing to a mating type threaded tubular connection (i.e., female if the threaded element beneath consideration is male and vice versa) to constitute a threaded tubular connection capable of resisting both static and cyclic stresses.
This threaded tubular element comprises a means which increases the flexibility of the first engaging threads and which thus aims at reducing the load transfer between the first engaging threads of the threaded tubular element and the last engaging threads of a mated threaded tubular element when these two elements form a threaded tubular connection subjected to tensile loads.
This means comprises a waist in the form of a groove on the wall of the threaded element starting from the peripheral surface opposite to that where the threading is formed. The groove does not affect the geometry of the threads, being formed between the envelope of the thread roots and the peripheral surface opposite that of the threading.
It has been formed in correspondence with the threading.
In accordance with the invention, the groove is such that, at the level of the first engaging threads, the thickness of the wall beneath the threading, i.e. the thickness of the wall measured starting from the thread root is reduced by the groove.
The function of this groove is to reduce the stiffness of the wall beneath the threading at the level of the first engaging threads, the stiffness of the wall varying with the stiffness of the wall beneath the threading.
Such a reduction in stiffness of the wall reduces the stiffness of the first engaging threads or increases their flexibility and thus reduces at their level the amount of the tensile load transfer. It there results a decrease in the stress peak at the level of the last engaging threads on a threaded element mated with the threaded element under consideration and made up in position with the latter so as to constitute a threaded tubular connection.
With a similar groove dimension, the inventor has established that the disposition of the groove according to the invention enables to optimize and ensure both static and dynamic (fatigue) operational characteristics of the threaded element under consideration when made up in position with a mated threaded element in a threaded tubular connection.
Preferably the groove starts beneath the first engaging thread of the threading.
Preferably the groove ends in an axial interval comprised between a cross section in the middle of the threading and a cross section situated at the level of the last engaging threads.
Preferably, the groove of the invention is a body of revolution around the axis of the threaded tubular element in the same transverse cross section. It thus reduces the stiffness circumferentially in the same manner around the threaded tubular element.
Preferably again, the groove of the invention reduces the stiffness of the wall which supports the threads perpendicularly thereunder in a varying but gradual manner in an axial direction of the threaded tubular element.
Such a disposition avoids a local concentration of stresses which may destroy the beneficial effect of reducing the stiffness of the structure on load transfer, and even initiate fatigue ruptures.
At the level of the groove of the invention, the thickness of the wall beneath the threading is minimum in a transverse plane preferably located in an interval between the first and sixth engaging threads.
Highly preferably, this minimum wall thickness beneath the threading is greater than or equal to the thread height and advantageously roughly equal to twice the thread height.
The idea to form such a deep groove is in opposition to the prevailing opinion which aims at reinforcing the ruggedness of the threaded elements taking into account their use in conditions often difficult on the drill sites and the stresses to be supported during operation in spite of a limited thickness.
Highly preferably again, the thickness of the wall beneath the threading at the level of the groove is minimum and constant on a non zero axial length.
Highly preferably again, because of the groove, the wall thickness beneath the threading is in the range 100% to 120% of the minimum thickness of the wall beneath the threading in a zone termed the xe2x80x9clow stiffness zonexe2x80x9d
This low stiffness zone is disposed around the transverse plane of minimum thickness of the wall beneath the threading and extends over an axial length equal to three or more times the thread pitch.
This enables a substantially maximum reduction in stiffness over an interval sufficient to allow for real variations in positioning of the first engaging threads with respect to the design because of the machining tolerances on the threaded tubular element.
Preferably, the profile of the groove is such that the critical section of the threaded element in axial efforts in particular tensile loads is located outside the waist. Preferably again, because of the groove, the thickness of the wall beneath the threading at the level of the last three engaging threads is in the range 80% to 100% of that of the non-waisted zone. The whole of the tensile load must in fact be absorbed by the critical cross section of the wall at the level of the last thread of the threading.
Preferably, the groove comprisesflanks the inclination of which with respect to the axis of the threaded tubular element is 45xc2x0 or less.
Preferably, the groove flank directed towards the side of the free end of the threaded element is globally more inclined than the other groove flank with respect to the axis of the threaded element.
Preferably again, the profile of the groove is a curve constituted by a series of arcs of a circle with a finite or infinite radius joined tangentially to each other, the arcs of a circle with infinite radius corresponding to straight line segments.
Preferably again, the groove joins the current unhallowed portion of the peripheral surface in which it is formed via a tangential junction zone which is toric in shape.
In a variation, the groove can be partially or completely filled with a material with an elastic modulus which is lower than that of the threaded tubular element.
Preferably again, whatever the embodiment of the groove of the invention, the threads are trapezoidal in shape.
The invention also concerns a threaded tubular connection with high resistance to static and cyclic stresses, comprising a male threaded tubular element at the end of a first pipe connected by screwing to a female threaded tubular element at the end of a second pipe by means of a male threading on the male threaded tubular element and a female threading on the female threaded tubular element.
The term xe2x80x9cpipexe2x80x9d means both a great length pipe and a short pipe such as a coupling.
Advantageously, at least one of the two male or female threaded tubular elements is of the type with the waist of the invention.
Highly advantageously from the viewpoint of performance under cyclic stresses, the two male and female elements are of the type with the waist of the invention.
Other advantages and characteristics of the invention will become clear from the detailed description below and from the accompanying drawings. This detailed description and accompanying drawings will not only serve to clarify comprehension of the invention but also contribute to the definition thereof, as appropriate.